Earth’s first living organisms didn’t leave behind footprints or bite marks or bones. These single cells thrived quietly in a tiny pocket somewhere on the planet. For centuries, scientists trying to describe this earliest life have relied on evidence provided by biology, studying what features modern life-forms have in common to deduce the most primitive components of cells. By working backward, biologists have developed proposals describing when and where such simple forms of life could have arisen. But the ideas so far are guesses at best, impossible to prove.
Researchers from a different field — geology — have more recently joined in the effort. With guidance from biologists, geologists are looking to Earth’s oldest rocks to uncover traces of life left behind by the very first cells. Geologists are also pointing biologists toward unusual environments where early cells might have gained a foothold. Where the two fields intersect, more concrete scenarios regarding life’s formative years are now taking shape.
Life, by definition, alters its surroundings, exchanging energy and chemicals with the world around it. So early cells should have left indelible chemical traces of their existence — clusters of elements that would never have come together without help from a metabolizing organism. Today, materials that could contain chemical signatures of Earth’s earliest life are few and far between, mostly buried deep within the planet’s interior, occasionally pushed to the surface when volcanoes erupt or mountains form. But geologists are determined to find and analyze these rocks for signs of life.
“The geological record is like a rug in an old house,” says Stephen Mojzsis of the University of Colorado Boulder. “Over centuries of people walking over it, it gets completely worn away and all you have is a few colored threads left. But if you look closely enough at those few threads, you might be able — thread by thread — to figure out what the rug once looked like.”
Finding the threads left by early life is only part of the challenge. Geologists are also contributing to discussions about where those threads were spun. One story, based on the discovery of a new type of underwater vent, begins deep in the ocean. Another proposal gets its start in vapor-fed ponds, where geologists say the requirements biologists have laid out for life’s existence could have been met.
As the collaboration plays out, geologists and biologists are providing a reality check for each other, determining what was possible.
“There’s a need for the biologists who are thinking about this to think in terms of real Earth processes and conditions rather than what they can do in a test tube, because clearly there weren’t test tubes lying around on early Earth,” says geophysicist Norm Sleep of Stanford University. “I consider it my duty to provide a shopping list of early environments to these biologists.”
Rocks of life
Four and a half billion years ago, the infant Earth was a hot and volatile place. Shortly after its formation, within 150 million years or so, it is thought to have been hit by a smaller young planet. The collision formed the moon and altered the Earth dramatically.
“There are bookends to the possible time for life to emerge on the planet,” Mojzsis says. “And one bookend is the formation of the moon — this was such a catastrophic event that it remelted the Earth and reset everything. There’s no way any life could have survived this.” But as the Earth cooled through this period, called the Hadean eon, it slowly became a more habitable place. By the start of the Archean, 3.8 billion years ago, life was thriving.
Geologists know that life existed then because rocks from the Archean have high concentrations of carbon. Before living organisms were around, most of the carbon on the planet was in the atmosphere. But the chemical reactions used to generate energy by photosynthetic organisms integrate carbon into the solid matter of the planet. So the presence of carbon-rich rocks mean photosynthesis was occurring. And it’s not just carbon. Ancient rocks with “banded iron formations” — red layers rich in iron — also indicate photosynthesis; the oxygen released by photosynthesis rusted iron on the planet’s surface (SN: 6/20/09, p. 24).
“We see this evidence of photosynthesis at 3.8 billion years ago,” says Stanford geochemist Dennis Bird. “But photosynthesis is an advanced chemical reaction, so life must have originated some time before that.” The trouble, he says, is the further back you go in Earth’s history, the less concrete the data.
The picture becomes fuzzier because rocks from the Hadean are hard to come by. Throughout the last 4 billion years, all the rocks and minerals on Earth have been cycling throughout different layers of the planet. Today, Earth’s oldest rocks are mostly contained deep within the mantle, a viscous layer of material that begins several kilometers beneath the surface and reaches temperatures of thousands of degrees. Rocks of all different ages and places of origin melt together in the mantle, the planet’s geological mixing bowl. By the time the rocks return to the surface thousands or billions of years later — in a volcano or ocean vent — they rarely resemble their old form.
“Basically all the direct evidence that we have from the Hadean is a collection of crystals that you could fit on the tip of a thimble,” Sleep says.
These crystals are mostly zircon, a durable mineral that keeps its form even in the hot turmoil of the mantle. Grains of ancient zircon have been found lodged inside other, newer rocks in Western Australia (SN: 6/18/83, p. 389). By studying the types and levels of oxygen and titanium ingrained in the zircons’ structures, scientists can tell at what temperature the crystals developed and whether they had contact with water. Zircons have even helped clarify the point in Earth’s history when continents formed.
Even when geologists discover ancient rocks dated to the Hadean, searching for signs of life within is tricky. Depending on how early cells functioned, and where, they could have left behind drastically different chemical clues. Sleep and Bird have recently conducted a comprehensive analysis of what the geological signs of early life from more than 3.8 billion years ago could be. Now, they want other geologists to keep an eye out for these signatures.
“Petrologists are studying the mantle all the time,” Sleep says. “But they’re not trained as paleontologists, so they toss away rocks as insignificant that we might find fascinating.”
Among the chemicals in rocks that could be key signatures of ancient life: sulfate, iron, uranium, nickel and nitrogen, the scientists propose in a recent paper in the Annual Review of Earth and Planetary Sciences. The geologists developed this list by consulting with biologists on what types of metabolisms enabled such organisms to grow and how they could reproduce and respond to their surroundings. Then the geologists used this information to piece together what traces could have been left behind. Such signatures, the team reports, could have survived the turmoil of the rock cycle since the Hadean.
Just the presence of these elements doesn’t mean life, but clusters of any one could be a clue.
“We’re conditioned to see what we’re looking for, and if no one knows what to look for in terms of these signs of life, they won’t find them,” says evolutionary biologist Bill Martin of Heinrich-Heine-Universität Düsseldorf in Germany. “What I think is really exciting is that we know what to look for now, and we have expectations of finding these biosignatures that could extend back to the Hadean.”
Scientists already have a good idea of where to look — Western Australia and the southwest coast of Greenland are famous for being sources of rocks more than 3 billion years old. And some areas of the globe have older material near the surface. Once they get through the slow process of finding the rocks, researchers can help narrow down the date of the origin of life on Earth. The window of possibility still extends almost a billion years.
But the knowledge to be gained goes beyond age. The chemicals left in rocks might reveal how early cells survived, or even point to the environments in which they thrived.
Most biologists believe early life arose in water, since all life today relies on some form of liquid for molecules to interact. But whether the first cells emerged in salty oceans or freshwater pools is a much-debated question. Geologists can help by cataloging environments that the early Earth hosted and describing the chemistry at play.
A chance discovery in 2000 added evidence to the idea that life began in the ocean. Scientists on a deep-sea expedition discovered a new kind of vent on the ocean floor in the middle of the Atlantic. After seeing the chemical and geological analyses from these Lost City vents, which support many microscopic life-forms, biologists including Martin realized that such spots could be a prime setting for early life.
While most deep-sea vents are driven by volcanic heat under the ocean floor, and create an acidic environment in the surrounding ocean, the Lost City vents are formed by a reaction between mantle rocks and seawater, leading to an alkaline environment.
The vents spew methane and hydrogen into the water, which react to form limestone chimneys, acetate (a potential energy source for early life) and hydrocarbons (important building blocks for life). What’s more, pores in the limestone chimneys suggest a way for chemicals to undergo reactions without floating away.
“These microcompartments serve the function of providing a way for chemicals to be concentrated in a physical way without cellular membranes,” explains Martin, who has collaborated with geologists to propose theories on the origins of life at the Lost City vents. Early cells could have lived off the chemical cocktails within the compartments. “Lost City is the most exciting thing that’s happened in the past decade in this field,” he says.
While today’s Lost City vents wouldn’t have been around 4 billion years ago, similar ones could have spewed life-sustaining chemicals into the early oceans. It’s up to geologists to determine whether such vents existed and up to biologists to figure out whether life could have thrived there.
Armen Mulkidjanian of the Universität Osnabrück in Germany has a different idea about where life began. He studies what features modern single-celled organisms share with each other, a favorite topic among many evolutionary biologists. Two major domains of such organisms exist — bacteria and archaea — and both are thought to have evolved from a common ancestor that existed at least 3.5 billion years ago.
While Earth’s first cells were probably more primitive than this ancestor, studying its characteristics may help scientists piece together a picture of even older cells. So far, researchers have found that bacteria and archaea have about 60 genes in common. Thus, scientists deduce, the ancestor, dubbed LUCA (short for Last Universal Common Ancestor), had these same genes.
“What we decided to do was to analyze what organic ions are required by each of the proteins coded by these ubiquitous genes,” Mulkidjanian says. His team’s analysis, reported in Proceedings of the National Academy of Sciences earlier this year, revealed that these shared proteins require potassium more than any other element. And sodium, the researchers showed, blocked the function of some of the cellular elements, most of which are involved in the translation of genetic material to proteins.
“We know that original membranes were very leaky. Cells could keep proteins or nucleotides inside, but not potassium,” Mulkidjanian says. So this means that LUCA must have been living somewhere with more potassium than sodium; otherwise potassium would have flooded right out of the membrane.
Here’s when the biologists consulted with geologists. Researchers familiar with ancient geology agreed that all the evidence from Hadean rocks suggests the oceans back then were rich in sodium. But one place on the ancient Earth, the geologists said, would be replete with potassium: ponds created by vapor from volcanic systems, which the Hadean planet had plenty of.
When magma from volcanoes heats rocks, some water evaporates, pulling certain elements from the rocks and leaving behind others. The resulting vapor can condense back into water and form freshwater ponds, rich not only in potassium but also in zinc and phosphate — also substances that could have driven early cellular processes. The cellular requirements of LUCA proposed by Mulkidjanian’s team matched the geological descriptions of these geothermal fields. “That geochemical knowledge is really what fed our biology story,” says Mulkidjanian.
Ideas about where life began, whether it was in an ocean or a pond or somewhere else entirely, are still just proposals, hypotheses with bits of evidence. The same is true for existing views about when life emerged and what it looked like. But as geologists and biologists continue to learn from each other, they’re turning up new evidence that can strengthen existing scenarios and lead to new ones.
For geologists, the challenge going forward is to find and analyze more ancient rocks to flesh out the picture of the Hadean planet. For biologists, the next task is to combine their theories on early cells with geologists’ descriptions of the early Earth.
“We’re starting to narrow the gap between microbiology and geochemistry,” Martin says.
As new zircons are uncovered and chemically favorable environments are explored, the tale of how life began may gain an agreed upon time, a setting and, eventually, a plot.
Sarah C.P. Williams is a freelance science writer based in Hawaii.